A spatial multiplexing scheme is used in optical communication techniques such as in conventional optical multi-chip modules (MCM) to increase the number of channels and expand the signal band. Often, an array is used which includes light-emitting elements used to transmit light, such as a vertical cavity surface emitting laser (VCSEL) with 12 channels and a 250 μm pitch, and light-receiving elements used to receive light, such as photodiodes (PD). A VCSEL/PD chip is mounted on a 12-channel optical waveguide, but further densification is being considered in order to obtain a wider bandwidth, such as 24 channels and a pitch of 125 μm, and 48 channels and a pitch of 62.5 μm.
It is assumed that the optical waveguide is connected to an optical fiber. Therefore, densification is limited to a pitch of 125 μm considering the fact that the clad diameter of optical fibers currently in use is 125 μm. Even if the limitation to a 125-μm pitch were overcome by reducing the diameter of optical fibers, densification of 48 channels or more limits the optical waveguide to a single layer considering the fact that core width of multi-mode optical fibers is 35 μm and light leakage occurs. When the optical waveguide has two or more layers, the connection loss due to a wider light beam becomes a serious problem.
FIG. 1 (Prior Art) is a top view schematically illustrating a densified optical module 100 using a spatial multiplexing scheme of the prior art. A plurality of optical waveguides 110 is arranged densely on the surface of a substrate 105. A single optical input/output unit 115 embodied by a reflecting means such as a mirror inclined by 45 degrees to reflect and change light from a horizontal direction to a vertical direction is provided on one end of each optical waveguide 110, and these units are not arranged side-by-side but separated from each other by some distance. Two electric pads 120, an input pad and an output pad, are provided for each optical input/output unit 115 to complete the VCSEL/PD chip 125. When the optical waveguides 110 have a width of 35 μm and are arranged at a pitch of 62.5 μm, the space between optical waveguides 110 is 27.5 μm. Even when light leakage is taken into account, it is difficult to connect electrical wiring from the electric pads 120 in a manner that does not adversely affect the arrangement of the optical waveguides 110.
A 500-Gbps parallel wavelength division multiplexing (PWDM) optical interconnect is disclosed in Non-Patent Literature 1 (B. E. Lemoff et al., 500-Gbps Parallel-WDM Optical Interconnect, Proceedings Electronic Components and Technology Conference, Vol. 2, 2005, pp. 1027-1031) in which 10.42 Gbps, 48-channel data transmission occurs via a ribbon having twelve parallel optical fibers with four wavelengths per optical fiber. VCSELs and PDs are connected to the optical fibers using coarse wavelength division multiplexing to provide densification. However, the propagation of light is controlled in this optical interconnect by reflection alone, and there is no structure for controlling the propagation of light in the waveguides. Also, the insertion loss at both the transmitter and the receiver end (from the light-emitter or receiver to the optical fiber) is a significant 6-8 dB.
An optical receiver is described in Laid-open Patent Publication No. 2011-257476 in which a first substrate made of an optically transparent material having a plurality of light-receiving elements formed in the obverse surface and a plurality of V-shaped grooves in the reverse surface and a second substrate made of an optically transparent material having the same refractive index as the first substrate and having a plurality of protrusions with a shape corresponding to the V-shaped grooves formed on the obverse surface are integrally formed by mating the V-shaped grooves into the protrusions and bonding them together, the wavelength-multiplexed light passing through the mated V-shaped grooves and protrusions passes through a non-reflective film formed on one of the inclined surfaces of each V-shaped groove without any reflection, and only light of the corresponding wavelength is reflected by a band reflection filter formed on the other inclined surface of each V-shaped groove towards a light-receiving element via the first substrate. Because light propagates through the first substrate and the second substrate, the first substrate and the second substrate are not optical waveguides. Therefore, a non-reflective film has to be formed on one of the inclined surfaces of each V-shaped groove to propagate the light.